Frequency hopping method for a mobile radio telephone system

ABSTRACT

A frequency hopping method for a mobile radio system by which the carrier frequency of the base station ( 11 ) and of one or more mobile stations ( 12 ) of the mobile radio system is temporally changed in defined intervals according to a pre-determined frequency hopping scheme. Preferably, certain operational conditions of the mobile radio system are monitored and the frequency hopping scheme is adaptively adjusted according thereto. As a result, during operation, a frequency hopping sequence can be adjusted to the present operational conditions, especially to the influence of possible interfering signal.

BACKGROUND OF THE INVENTION

The present invention relates to a frequency hopping method for a mobileradio system, especially for a WDCT mobile radio system.

Frequency hopping is used as is known in communication systems,especially in mobile radio systems to improve the transmissionreliability. For this purpose the carrier frequency is changed indefined intervals, whereby in a mobile radio system the mobile stationsare prompted by the base station to hop frequencies. Equally thesequence of the frequency hopping, that is to say the sequence of thecarrier frequencies to be used after each other is notified to themobile stations by the base station.

A known mobile radio standard, in which for example frequency hopping isused, is the so-called DECT (Digital European Cordless Telephone) mobileradio standard. This mobile radio standard was developed by ETSI(European Telecommunications Standard Institute), in order to have aleading European system also available for so-called CT (CordlessTelephone) applications, that is to say for cordless telephony. TheDECT-mobile radio standard provides very good transmission quality butwith limited range. The use of cordless DECT-systems is thereforepreferred in domestic homes or in business premises.

In the case of the DECT-mobile radio standard ten different carrierfrequencies with an interval of 1728 kHz are used for frequency hopping.The frequency spectrum in total ranges from 1880 to 1900 MHz. Thecarrier frequencies to be used for frequency hopping are notified to themobile stations by the base station via a control channel. For thispurpose a control message is transmitted by the base station to themobile stations, which informs the mobile stations of the carrierfrequencies available to the base station for frequency hopping. Thismessage covers a bit field where for ten possible carrier frequencies abit is optionally set in each case to ‘1’ or ‘0’ by the base station. Ifthe bit corresponding to a defined carrier frequency is set to ‘1’, thismeans for the mobile station that the corresponding carrier frequencycan be used for frequency hopping.

With the previously described principle frequency hopping is limited toa maximum of ten different carrier frequencies. The sequence of thecarrier frequencies to be used for frequency hopping is pre-determined.If during a transmission for example a disturbance occurs due to acollision with interference frequencies, a new transmission using thesame frequency hopping sequence is attempted which however often failsif the same disturbance (for example microwave frequencies) stillcontinue.

Equally in further later standards, like for example Bluetooth or theSWAP Standard, static frequency hopping methods are used, with the aidof which collisions with interfering signals also cannot be reliablyprevented and as a result the performance capability of the system isinevitably impaired.

Furthermore the DECT standard was essentially developed for the Europeantelephone market. Since however the need for a reliable cordlesstelephone system also exists in the American market the so-called WDCT(Worldwide Digital Cordless Telephone) mobile radio standard wasdeveloped by the applicant based on the DECT standard. The WDCT standarduses a frequency band of between 2400 and 2483.5 MHZ which is compatiblewith the American FCC provisions for unlicensed operation.

As well as the frequency band being used according to the FCC provisionsfurther conditions however are imposed on the mobile radio system beingoperated which amongst other things also concern the frequency hoppingmethod used in each case. Thus the FCC provisions require that thefrequency hopping method must encompass at least 75 different carrierfrequencies since each frequency must not be used or maintained forlonger than 0.4 sec within a period of 30 secs. Each frequency must onaverage be used the same number of times. Further it is required thatthe carrier frequencies being used in each case must be selected from apseudo randomly arranged carrier frequency list.

SUMMARY OF THE INVENTION

The present invention is generally based on the aim of proposing afrequency hopping method for a mobile radio system with which moresecure operation and better transmission reliability can be achieved.Especially the frequency hopping method should make it possible to meetthe aforementioned FCC provisions as well as frequency hopping based onthe DECT standard with as little change to the DECT protocol aspossible.

The aforementioned aim is achieved according to the present invention bya frequency hopping method.

According to the invention defined operational conditions of the mobileradio system, especially the presence of possible noisy carrierfrequency channels, are monitored and the pre-determined frequencyhopping sequence is adapted accordingly thereto. Preferably depending onthe aforementioned monitoring the mobile stations are notified which ofthe carrier frequencies intended according to the pre-determinedfrequency hopping sequence should not be used.

The present invention therefore allows the use, optimally adapted to theparticular operational conditions, of a large number of carrierfrequencies, with the individual mobile stations being notified in eachcase by the base station which of the carrier frequencies are to beused. In particular with the help of the present invention compliancewith the FCC provisions described above is possible.

The update information about which of the carrier frequencies should notbe used is transmitted via a control channel of the mobile radio system.The mobile radio system preferred is in the form of a WDCT mobile radiosystem based on the DECT standard configured with 95 possible carrierfrequencies in the frequency range of between approx 2400 and 2500 MHz,whereby the previously mentioned update information can be transmittedvia the N_(T) or Q_(T) control channel. For this purpose correspondingchanges of the DECT protocol are proposed in accordance with certainembodiments of the invention.

The change of the carrier frequency can be implemented both from frameto frame as well as from time slot to time slot, that is to say within aframe.

The invention is described below in more detail by way of preferredembodiments with reference to the attached drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a status diagram to explain the conditions occurring with amobile radio according to the invention and status changes of the basestation,

FIG. 2 shows a status diagram to explain the conditions occurring with amobile radio system according to the invention and status changes of amobile station,

FIG. 3 shows a comparison of the N_(T) message transmitted with aconventional DECT mobile radio system and a WDCT mobile radio systemaccording to a first embodiment of the present invention,

FIG. 4 shows a comparison of the Q_(T) message transmitted with aconventional DECT mobile radio system and a WDCT mobile radio systemaccording to a second embodiment of the present invention,

FIG. 5 shows a comparison of a section of the P_(T) message transmittedwith a conventional DECT mobile radio system and a WDCT mobile radiosystem in accordance with a further embodiment of the present invention,

FIG. 6 shows the structure of a frame transmitted according to the DECTmobile radio standard,

FIG. 7 shows the transmission and priority of control channels in amulti-frame structure, and

FIG. 8 shows a simplified block circuit diagram of a mobile radio systemaccording to the invention.

The present invention is explained below by way of the WDCT mobile radiomethod developed by the applicant whereby however the invention canessentially also be applied to other telecommunication standards.

The WDCT mobile radio standard is as already explained based on theknown DECT mobile radio standard.

In FIG. 6 the frame and time slot structure for a transmission signal isillustrated according to the DECT standard. As already explained withthe DECT standard the available frequency spectrum (1880–1900 MHz) isdivided into ten carriers each of 1728 KHz.

As shown in FIG. 6 each carrier frame 8 is divided into 24 time slots of480 bits (time slot duration 416.6 μsec) whereby for the downlink, thatis to say for a transmission from the base station to a mobile station,the first 12 and for the uplink, that is to say for a transmission froma mobile station to the base station, the second 12 time slots areprovided. For a duplex connection pairs of time slots are formed,whereby for example the time slot 11 and the time slot 23 form a pair.In this way 12×10=120 duplex channels with a transmission rate of 1152kbit/s are available.

The duration of the DECT frame shown in FIG. 6 is 10 ms, whereby every10 ms in one time slot 9 388 bits which are available for the so-calledMAC (Medium Access Control) Layer are transmitted once in a time slot.The main task of the MAC Layer is resource management, multiplexing ofdifferent control channels as well as protection against erroneoustransmission.

As shown further in FIG. 6 the 388 bits intended for the MAC Layerencompass an S-field (32 bits) for synchronisation purposes, an A-field(64 bits) for control and signalling purposes, a B-field (40 bits) asuseful data field and four spare bits. The corresponding time slot 9 isfilled by a protection band (guardband) encompassing 60 bits.

FIG. 6 also shows the structure of the aforementioned A field 10. The 64bits of the A field 10 are divided into a header (8 bits), certaincontrol channels (40 bits) and a CRC (Cyclic Redundancy Check, 16 bits)field. The CRC field serves to detect transmission errors on thesignalling or control channels. In each A field one or several logiccontrol channels Q_(T), P_(T), C_(T), N_(T) and M_(T) are transmitted inmultiplexed form. Here the 40 bits of the A field available are dividedin each case depending on current demand between the individual controlchannels whereby a priority scheme shown in FIG. 7 and embedded into amulti-frame structure encompassing 16 DECT frames 8 (0 . . . 15) isused. Thus for example the control channel CT is only transmitted in theA-field for example from the base station of the corresponding DECTmobile radio system in the frame No. 1 unless the MT control channelmust be transmitted. In addition for example the control channel CT isonly transmitted in the corresponding A field by a mobile station inthis DECT mobile radio system unless either the control channel MT orthe control channel NT must be transmitted. Thus in accordance with thepriority scheme shown in FIG. 7 the assignment of the A fields of theindividual control frames 8 is fixed temporally multiplexed both for theuplink as well as for the downlink.

Based on this known structure of the A field of a DECT frame, changesare proposed in accordance with the preferred embodiments of the presentinvention which enable update information to be transmitted in order tocontinually adapt the freqency hopping sequence, pre-determined by thebase station at the beginning of the operation and subsequently storedpreferably in the individual mobile stations in the form of a frequencyhopping list, to the particular conditions so that it is ensured thatonly those frequencies are used for frequency hopping in which nocollisions with interfering signals occur. In accordance with theinvention therefore an adaptive frequency hopping strategy is proposedwhich enables collisions with interference frequencies to becircumvented or avoided.

In FIG. 8 the basic structure of a mobile radio system operated forexample in accordance with the WDCT mobile radio standard with a basestation 11 and several mobile stations 12 communicating with it areshown in a greatly simplified way. Both the base station 11 as well asthe individual mobile stations 12 in each case have a WDCT controller 13or 15, which is configured in such a way that in the frequency band2400–2483.5 MHz, a cordless telephone operation is implemented using afrequency hopping method compatible with the US FCC provisions.Implementation of the frequency hopping strategy or frequency hoppingmethod can in principle be left to the operator, whereby the sequence tobe used during hopping of the carrier frequencies is specified by thebase station 11 and transmitted to the mobile stations 12. Thisfrequency hopping list is stored both in the base station 11 as well asin the mobile stations 12 in a ROM memory 14 or 16. During operation ofthe mobile radio system shown in FIG. 8, update information ispreferably transmitted continuously to the mobile stations 12 by thebase station 11 which notifies the mobile stations 12 which of thecarrier frequencies stored in the frequency hopping list should not beused in future, in order to be able to take into consideration parasiteinfluences occurring with certain carrier frequencies. For this purposethe frequency environment of the mobile radio system can be continuouslymonitored by the WDCT controller 13 of the base station 11 for possibleinterference frequencies or an updated frequency hopping list can begiven by the operator of the mobile radio system to the base station 11.

To transmit this update information relating to the frequency hoppinglist or frequency hopping sequence stored in the mobile stations 12 butespecially to meet the FCC provisions already mentioned, changes to theDECT-MAC Layer or the A field are required. These changes are made inaccordance with the preferred embodiments of the present inventionexplained below especially in such a way that as little change to theDECT protocol as possible is necessary, so that the higher layers can betaken over by the DECT standard.

In the case of the WDCT mobile radio system shown in FIG. 8, 95 carrierfrequencies are available in the frequency range of 2400 to 2483.5 MHz.The FCC provisions for using frequency hopping require at least 75different carrier frequencies so that to avoid disturbances orcollisions with interference frequencies a maximum of 20 availablecarrier frequencies cannot be used. The frequency hopping sequenceactually used corresponds to a sub-set of the 95 frequencies available.For each base station 11 for example depending on the identificationsignal or ID of this base station a separate pseudo-randomly arrangedlist of these 95 carrier frequencies can be produced, keeping channelseparation to a minimum, in order not to lose more than one carrier dueto wide band interference. The frequency hopping list produced in thisway is as already mentioned known both to the base station 11 as well asto all mobile stations 12 communicating with the latter.

If the WDCT Controller 13 of the base station 11 detects a noisychannel, that is to say a carrier frequency, in which no transmission ispossible with sufficient quality, an update of the previously storedfrequency hopping list or sequence is provided in order to eliminatethese noisy channels from the frequency hopping cycle. In accordancewith the invention therefore this involves an adaptive frequency hoppingmethod.

Two frequency hopping concepts are described below whereby with thefirst concept the carrier frequency is changed from time slot to timeslot, so that for different mobile stations 12 various frequency hoppingsequences can be implemented which results in a very large amount offlexibility with greater control requirement. On the contrary accordingto the second concept it is proposed only to change the carrierfrequency from frame to frame so that all mobile stations 12synchronised to the same base station 11 must be operated with the samefrequency hopping sequence. This second concept is more easy toimplement but is less flexible.

The frequency hopping methods used must be designed in such a way thatthe different operational conditions of the mobile stations 12 and thebase station 11 as well as the status changes possible in each case aretaken into consideration. This will be explained in more detail below byway of the illustrations in FIG. 1 and FIG. 2.

FIG. 1 shows the different active operational conditions of the basestation 11 whereby the base station 11 is in an operational state 1described as “active idle” unless there is a connection to a mobilestation 12. In this state a so-called “dummy bearer” signal is sent bythe base station 11 and at the same time periodically all physicalchannels available are scanned in order to detect a connection enquirysent by a mobile station 12. As soon as a connection to a mobile station12 is built up, the base station 11 changes to a state 2 described as“active traffic”. It again changes to state 1 if all connections havebroken up and a dummy bearer signal is again sent by the base station11. The base station 11 changes from state 2 into state 3 described as“active traffic and idle” if a dummy bearer signal is also transmittedby it in addition to at least one “traffic bearer signal”. Return fromstate 3 to state 2 occurs if no further dummy bearer signal is sent bythe base station 11. Accordingly base station 11 changes from state 1 tostate 3 as soon as at least one connection is built up, while a returnfrom state 3 to state 1 takes place, if the last connection with amobile station 12 has also broken up.

FIG. 2 shows the different operational states of a mobile station 12whereby the mobile station 12 is not active in a state 4 described as“idle unlocked”. Directly after being connected the mobile station 12changes to a state 5 described as “active unlocked”. In this state themobile station 12 tries to synchronise to the base station 11, all timeslots with a fixed frequency being scanned. If for example no dummybearer signal has been detected by the base station 11 within a periodof one second, the frequency is changed. If the mobile station 12 hasdetected the dummy bearer transmitted by a base station 11 both a bit aswell as a time slot synchonisation to the base station 11 is carried outautomatically with the aid of the software and hardware used in theparticular case. For this purpose the messages transmitted in each casein the A-field are evaluated. In contrast to the known DECT standard inwhich a fixed frequency hopping sequence is always used, synchronisationto the current valid frequency hopping sequence of the base station 11must also be carried out in addition. After the synchronisationprocesses have been carried out the mobile station 12 changes to a state6 described as “idle locked”.

In this state the mobile station 12 can be operated in a sleep mode byentering a corresponding software command to reduce power consumption,whereby in this case the mobile station 12 or its controller 15 isactivated only every n-th frame in order to detect a dummy bearer signalwhereby n for example can be up to 64 for a sleep mode with minimumpulse width repetiton rate. The mobile station 12 can therefore remainsychronised with minimum power consumption to the base station 11. Thesleep mode can be left by the controller 15 or the correspondingprocessor in two different ways. Either a multi-frame structure ofmonitoring multi-frame meters shown in FIG. 7 has counted down to zeroor for example an external interrupt “waking up” of the processor isgenerated via the keyboard. After leaving the sleep mode the software ofthe corresponding mobile station 12 must be notified as to how long itwas in sleep mode in order to be able to again start synchronisationwith the frequency hopping sequence of the base station 11. For thisreason the present values of the multi-frame meter already mentioned andthe normal frame meter are known to the micro-controller 15.

As soon as a call is made by the mobile station 12 or the base station11 a traffic bearer signal is sent. Normally the dummy bearer alsocontinues. Then the mobile station 12 changes to a state 7 described as“active locked”. The base station 11 in this case changes to the state 3shown in FIG. 1 (“active traffic and idle”). The maximum number ofsimultaneous connections corresponds to the number of available timeslots in the transmission direction. It must be ensured that the dummybearer is released if a fourth connection is built up whereby the basestation 11 changes to the state 2 shown in FIG. 1 (“active traffic”).

As already mentioned both frequency hopping within a frame, that is tosay between individual time slots, as well as only between two frames ispossible in the context of the present invention.

With the first frequency hopping variant the carrier frequency must bechanged between two time slots of the same frame.

All 95 carrier frequencies of the frequency hopping list stored in thebase station are used to transmit the dummy bearer signal. This sequenceis not changed to transmit the dummy bearer and is also only used forthe dummy bearer. Since this sequence is also known to the mobilestations these can change their frequency after synchronisation to thebase station. This frequency hopping sequence is used by all mobilestations which are in the synchronised state 6 (“idle locked”) shown inFIG. 2. A mobile station in this state can, as already mentioned, beswitched over to a power saving sleep mode, whereby in this case themobile station is only activated in intervals of n frames to receive adummy bearer signal. The A field of the dummy bearer signal shown inFIG. 6 should as with the the DECT standard contain a P_(T) message inorder to be able to detect connection enquiries from the base station.This can be implemented by the fact that the first frame of themulti-frame shown in FIG. 7 is always received.

If on the other hand a mobile station is in state 7 (“active locked”)shown in FIG. 2 after a connection is built up, if interference occursthe frequency hopping sequence of the carrier signal should be changedor adapted. Since these changes of the frequency hopping sequence can beimplemented individually for each traffic bearer, each mobile stationmay possess a different frequency hopping sequence after adaption. Aswill be explained in more detail below the update information for thefrequency hopping sequence is transmitted for each traffic bearer withthe aid of the A field of the corresponding time slots. After aconnection has broken up the corresponding mobile station again changesto state 6 (“idle locked”) whereby the valid frequency hopping sequencefor this traffic bearer is also deactivated or broken up. Thecorresponding mobile station is then again sychronised to the dummybearer signal of the base station.

As soon as four connections are active simultaneously the dummy bearermust be deactivated. Since a maximum of six mobile stations can beoperated at the same time on one base station, this means that up to twomobile stations in state 6 (“idle locked”) can lose theirsynchronisation with the base station. In order to prevent this themobile stations in state 6 should be notified with the aid of the firstframe of the multi-frame structure shown in FIG. 7 that the dummy beareris deactivated. These mobile stations can then follow the last activatedtraffic bearer signal or its frequency hopping sequence whereby thefrequency hopping sequence of this bearer signal must not be changeduntil all mobile stations in state 6 (“idle locked”) have confirmed acorresponding request by the base station to change the frequencyhopping sequence. It must be ensured that during the timespan in which afourth traffic bearer signal is active, the mobile stations in state 6have increased power consumption since they must receive the controlfield or A field in each frame, that is to say switch over to sleep modeshould be avoided in this case. Should a connection again brake up sothat less than four traffic bearers are active simultaneously the dummybearer signal can be again activated and the mobile stations in state 6can again be changed over to the frequency hopping sequence of thisdummy bearer.

According to the second frequency hopping variant the carrier frequencymust only be changed at the beginning of a new frame, that is to say alltime slots of a frame are transmitted on the same carrier frequency.

Also with this variant of frequency hopping strategy described for afrequency change within a frame can be used in principle. As soon as atraffic bearer signal has been activated its frequency hopping sequenceis also adapted depending on the frequency environment of the mobileradio system. Since as a result the dummy bearer is also influenced, allmobile stations must be notified by an update of the frequency hoppingsequence in order to be able to carry out appropriate measuresthemselves. Thus in this case for example use of the sleep mode for themobile stations in state 6 (“idle locked”) can be dispensed with, sothat these can follow the frequency hopping sequence of the dummybearer. However this results in increased power consumption of thesemobile stations. If it is not possible to dispense with the sleep mode,this means that the mobile stations concerned only search in intervalsof n frames for the dummy bearer signal. In this case then the frequencyhopping sequence of the traffic bearer must not be changed until allmobile stations in state 6 have confirmed a corresponding request by thebase station as a result of which the timespan until the frequencyhopping sequence has been adapted is extended and as a resultsensitivity against parasite signals is increased. In the case of thesecond frequency hopping variant described previously in contrast tofrequency hopping within a frame when a second connection is built up,the first traffic bearer signal should be used for synchronisation, thatis to say the mobile stations in state 6 (“idle locked”) no longerchange their frequency in accord with the dummy bearer signal, but withthe first traffic bearer signal.

In the case of both frequency hopping variants described previously themobile stations are notified of the carrier frequencies in each casecurrently valid for frequency hopping via the A-field. The correspondingmessage should preferably be confirmed by the mobile stations concernedbefore the frequency hopping sequence is actually changed. Fortransmitting this update information concerning the frequency hoppingsequence adaption of the DECT-MAC layer already described using FIG. 6is necessary to implement the frequency hopping strategies described.This concerns especially the method as to how the update information canbe transmitted by the base station to the mobile stations. This updateinformation details, in view of possible collision with interferencefrequencies, which of the carrier frequencies intended in the originalfrequency hopping sequence should not currently be used.

With the DECT standard information about the available frequencychannels is transmitted with the aid of the Q_(T) message via the Afield. According to the WDCT standard however in comparison with theDECT standard a number of time slots and carrier frequencies deviatingfrom the latter are used so that adaption of the DECT protocol arenecessary especially anywhere where this information is transmitted.While the four bits intended according to the DECT standard to identifythe four time slot pairs are sufficient for the WDCT standard, this isnot true in the case of the ten bits provided according to the DECTstandard to identify the carrier frequencies. Therefore variouspossibilities to implement the present invention are indicated below.

According to a first embodiment of the present invention it is proposedthat the N_(T) message already provided according to the DECT protocolis used to transmit the previously mentioned update information.

FIG. 3A shows the division of the 40 bits of the A field shown in FIG. 6in the case of an N_(T) message according to the conventional DECTstandard. According to the DECT Standard the N_(T) message is dividedinto two sub-fields whereby the first sub-field covers so-called PARI(Primary Access Right Identifier) data and the second sub-field thelowest value bits of so-called RPN (Radio Fixed Part Number) data, PARIClasses A and B are used for the PARI data whereby in Class A the PARIdata includes EMC (Equipment Manufacturer's Code) data and FPN (FixedPart Nurnber) data, while in Class B the PARI data includes EIC(Equipment Installer's Code) data and the FPN data. As shown in FIG. 3Athe PARI data has 32–37 bits of the 40 bits available in total for theN_(T) message.

According to the first embodiment of the invention it is intended thatthe bit number provided for the PARI data is reduced especially by eightbits This can be achieved in Class A by reducing the EMC and/or the FPNdata and in Class B by reducing the EIC and/or FPN data. This is shownin FIG. 3B and the eight bits gained in this way then define thefrequency hopping data HOP. This frequency hopping data HOP can forexample specify which of the next eight carrier frequencies of thestored frequency hopping list must not be used.

Furthermore all available carrier frequencies can also be combined inblocks of for example twelve frequencies in each case, so that also inthis event only eight bits are sufficient to deactivate one of theseblocks and therefore all frequencies belonging to this block. As alreadymentioned according to the WDCT standard it is the intention to use all95 carrier frequencies for frequency hopping. This means that one blockonly has eleven frequencies. The available carrier frequencies shouldalways be grouped together so that if collisions with typical parasitesignals occur the best possible transmission quality is assured.

However only a maximum of two of the eight frequency blocks grouped inthis way should be eliminated from the pre-determined frequency hoppingsequence. In this case the eight bits of the frequency hopping data HOPindicate those blocks which should not used for frequency hoppingwhereby for example a bit of the HOP field is allocated to eachfrequency block and the corresponding frequency block is deactivated ifthe bit has the value ‘0’.

According to a second embodiment of the present invention it is proposedthat the Q_(T) message already provided according to the DECT protocolis used and changed to transmit the update information previouslymentioned.

FIG. 4A shows the format of the Q_(T) message used with the DECTstandard and described as “Static System Info” with the aid of which theavailable carrier frequency channels are notified to the mobile stationsby the base station. The Q_(T) message has the bits b8–b47 of thecorresponding A field (see FIG. 6). The four bits b12–bl5 define thenumber of that time slot pair (Slot Number, SN) at which thetransmission should start. The ten bits b22–b31 (RF Carrier bits, RFC)notify the mobile stations which of the fixed pre-determined carrierfrequencies are available at the base station. For this purpose acorresponding carrier frequency is allocated to each bit of the RFCfield whereby the carrier frequency can be used if the corresponding bithas the value ‘1’. In addition the channel number (Channel Number, CN)of the bearer of this transmission is encoded in the Q_(T) message. ThisCN data has the bits b34–b39. Finally the Q_(T) message also containsso-called PSCN (Primary Receiver Scan Carrier Number) data which detailsthat carrier frequency on which the receiver is operated in the nextframe. The PSCN data has the bits b42–b47. Furthermore the Q_(T) messagehas several spare bits (Spare bits, SP).

Since the WDCT standard uses 95 different carrier frequencies, theformat of the known Q_(T) message must be changed.

Thus several A fields with corresponding Q_(T) messages could be used todefine all 95 carrier frequencies. In the DECT standard a so-called“Extended RF Carrier Information” Q_(T) message is already defined bythe use of which only four of such Q_(T) messages would be necessary todescribe all 95 frequencies. As shown in FIG. 7 using the multi-framestructure illustrated there, the Q_(T) message however is onlytransmitted within one frame, namely frame No. 8 of this multi-framestructure. This means that when using this “Extended RF CarrierInformation” Q_(T) message to transmit the total carrier frequency datafour complete multi-frame structures will be necessary needing a totalof 4×160 ms=640 ms and as a result is not advantageous for fast updatingof the frequency hopping sequence.

For fast updating of the frequency hopping sequence therefore preferablymore simple methods should be used. Thus the ten available bits b22–b31of the RFC data can be used, similar to the change of the N_(T) messagealready explained by way of FIG. 3 to establish which of the nextcarrier frequencies of the pre-determined frequency hopping list or—ifthe frequencies are grouped into several blocks—which of the individualfrequency blocks should be used/activated or not used/deactivated. Whileaccording to the DECT standard the RFC field always involves the samecarrier frequencies, when using the present invention, the RFC fieldinvolves either eight frequency blocks or only the next ten carrierfrequencies of all 95 carrier frequencies.

As already mentioned the Q_(T) message is only suitable to a limitedextent for the WDCT standard to transmit the frequency hopping datasince for this purpose on the one hand a relatively long timespan isnecessary and on the other hand the frequency hopping data is identicalfor all mobile stations synchronised with the base station. The Q_(T)message should therefore only be used to transmit the update orfrequency hopping data or the frequency hopping variant in which thecarrier frequency remains constant within one frame. When using thefrequency hopping variant according to which the carrier frequency ischanged within one frame between two time slots, the frequency hoppingdata contained in the Q_(T) message should then be disregarded.

As shown in FIG. 4 an additional bit is needed for the WDCT standard toidentify one of the 95 available carrier frequency channels, that is tosay the CN field has the bits b34–b40. For this purpose one of the sparebits provided in the DECT standard is used. Equally for the WDCTstandard as shown in FIG. 4, the PSCN field is expanded by one bit inorder to take into consideration the larger carrier number in the caseof the WDCT standard whereby for this purpose the other of the sparebits present in the case of the DECT standard between the CN and thePSCN field is used. The PSCN field can however be disregarded since thisdata is already contained in the frequency hopping list.

For the WDCT standard the P_(T) message provided according to the DECTstandard should also be adapted. This message will then be received bythe mobile stations in state 6 (“idle locked”) themselves if these arein sleep mode and serves to transmit connection enquiries from the basestation. Furthermore some important data of the DECT-MAC layer istransmitted via this type of message.

Thus the P_(T) message for example has twelve bits, which in each caseare allocated to one of the twelve time slot pairs and notify thereceiving mobile station, if due to possible parasite influences thecorresponding time slot cannot be used. For this purpose the particulartime slot pair is marked accordingly so that the mobile station alwaysknows, via which time slots data can generally be expected. The sameconcept can also be applied to the WDCT standard, whereby however onlyfour bits are necessary for this purpose.

The P_(T) message is also used to transmit carrier data to the mobilestations, whereby this carrier data especially contains data about theother carriers, recommended carriers and dummy- or connection-lesscarrier positions. As shown in FIG. 5A this carrier data is transmittedencoded in the form of bits b36–b47 of the P_(T) message and has anSN-field with bits b36–b39 to describe the time slot number, spare bits(SP) b40, b41 and a CN-field with bits b42–b47 to describe the channelnumber. In the case of the WDCT standard in principle this format can betaken over, whereby using one of the two spare bits it is possible toexpand the CN-field for the channel number to eight bits b41–b47.Furthermore the SN-field can be reduced to two bits to describe the timeslot number.

With the aid of the modifications to the DECT-MAC layer described abovethe frequency hopping data, that is to say data about the carrierfrequencies used for frequency hopping, can be transmitted by the basestation to the mobile stations. In this case it must be ensured thatwhen using the N_(T) message to transmit this frequency hopping data theN_(T) message according to the priority scheme shown in FIG. 7 hasminimum priority and in the worst case is only transmitted in frame No.14. In this case synchronisation of a mobile station with the basestation can fail, if no N_(T) message is received and the next carrierfrequency of the stored frequency hopping list is automaticallydeactivated. This however is only critical if in the cases previouslydescribed the traffic carriers must be used to synchronise with the basestation.

1. A frequency hopping method for a mobile radio system, the mobileradio system having a base station with a carrier frequency and at leastone mobile station, each mobile station having a carrier frequency, themethod comprising: changing the carrier frequency of the base stationand the carrier frequency of at least one mobile station temporally indefined intervals according to a pre-determined frequency hoppingscheme; monitoring certain operational conditions of the mobile radiosystem; providing update information to adapt the frequency hoppingscheme to a frequency environment of the mobile radio system;transmitting the update information from the base station to each of theat least one mobile stations via a control channel of the mobile radiosystem and, further, indicating the carrier frequencies of the frequencyhopping scheme to be avoided for the frequency hopping method by each ofthe at least one mobile stations to each of the at least one mobilestations; and changing the carrier frequencies of the base station andeach of the at least one mobile stations according to the pre-determinedfrequency hopping scheme based on the update information; and wherebythe update information for the next n carrier frequencies according tothe pre-determined frequency hopping scheme of the at least one mobilestation indicates whether or not the corresponding carrier frequency forthe frequency hopping method should be used, or all carrier frequenciesavailable for the frequency hopping method are combined in severalfrequency groups, whereby the update information indicates for each ofthese frequency groups whether or not the carrier frequencies belongingto the particular frequency group should be used for the frequencyhopping method.
 2. The frequency hopping method according to claim 1,wherein the method further comprises storing pre-determined frequencyhopping scheme in the base station as well as in each of the at leastone mobile stations.
 3. The frequency hopping method according to claim1, wherein the pre-determined frequency hopping scheme has carrierfrequencies in a frequency range between approximately 2400 and 2500MHz.
 4. The frequency hopping method according to claim 1, wherein themethod further comprises arranging the frequency hopping scheme withseveral carrier frequencies according to a pre-determined and randomhopping sequencer; and uniformly spacing said carrier frequencies at adefined frequency interval, whereby a minimum frequency hopping intervalcovers six carrier frequency channels in the pre-determined and randomhopping sequence.
 5. The frequency hopping method according to claim 4,wherein the pre-determined frequency and random hopping scheme has 95different carrier frequencies.
 6. The frequency hopping method accordingto claim 1, wherein the step of providing update information includes:providing update information dependent on whether or not, whenoperational conditions of the mobile radio system are monitored, a noisycarrier frequency channel has been detected, and in the event that anoisy carrier frequency channel is detected, providing the updateinformation so that it indicates avoidance of corresponding carrierfrequency.
 7. The frequency hopping method according to claim 1, whereinthe step of transmitting the update information includes: embeddingcommunication data in a frame structure; and transmitting the embeddedcommunication data between the base station and each of the at least onemobile stations, whereby each frame of the frame structure coversseveral time slots, and the carrier frequency is changed betweenindividual time slots of a frame.
 8. The frequency hopping methodaccording to claim 1, wherein the step of transmitting the updateinformation includes: embedding communication data in a frame structure;transmitting the embedded communication data between the base stationand each of the at least one mobile stations , and maintaining thecarrier frequency constant within a frame but changing said carrierfrequency from frame-to-frame.
 9. The frequency hopping method accordingto claim 1, wherein the step of transmitting the update informationincludes: transmitting the update information via a N_(T) controlchannel, said N_(T) control channel having a PART field.
 10. Thefrequency hopping method according to claim 9, wherein the updateinformation is transmitted in a field covering eight bits (HOP), wherebythe PART field of the N_(T) control channel is reduced by these eightbits.
 11. The frequency hopping method according to claim 1, wherein thestep of transmitting the update information includes: transmitting theupdate information via a Q_(T) control channel.
 12. The frequencyhopping method according to claim 11, wherein the step of indicating thecarrier frequencies of the frequency hopping scheme to be avoidedincludes: indicating for each of the available carrier frequencies withthe aid of several Qr messages following on from each other, whetherthese should be used for the frequency hopping operation or not.
 13. Thefrequency hopping method according to claim 1, wherein the methodfurther comprises: combining all carrier frequencies available for thefrequency hopping method in several frequency groups, whereby the updateinformation for each of these frequency groups indicates whether or notthe carrier frequencies belonging to the particular frequency groupshould be used for the frequency hopping method , and uniformly dividingavailable carrier frequencies among the frequency groups so that, iftypical parasite influences occur, resulting in noisy carrier frequencychannels, the best possible transmission quality is assured.
 14. Thefrequency hopping method according to claim 1, wherein the updateinformation is transmitted via a control channel.
 15. The frequencyhopping method according to claim 14, wherein the control channelincludes a format that is based on a MAC layer of a DECT mobile radiostandard.